It is often necessary to closely monitor or sense the current flow through some critical circuit paths in order to make appropriate power management decisions and to maximize the performance of the circuit. One method of sensing current is to add a resistor in series with the current path. The voltage drop across the resistor is proportional to the current flow through the resistor. The added resistor is inefficient in that a resistance is added to the circuit, which reduces the current flow and increases power losses.
One method of sensing current through a power transistor, such as a power metal oxide semiconductor field-effect transistor (MOSFET) uses SenseFET technology. A SenseFET circuit is essentially comprised of two matched transistors, such as two MOSFETs. One transistor is the main current-carrying power transistor and the other transistor is a much smaller sensing transistor. The circuit includes a small number of source cells that are isolated from the rest of the source cells and connected to a separate sense pin, which creates a matched mirror transistor. The ratio of source cells to sense cells is typically very large, so when current flows through the main transistor of the source cells, a much smaller current flows through the mirror transistor. The ratio of current through the main transistor to the current through the mirror transistor is referred to as the current ratio, and is typically about 400:1. The current flow through the sensed transistor is measured, such as by measuring the drain/source voltage, which provides an indication of the current flow through the main transistor. SenseFET technology is more efficient than a current sense resistor, but is limited by the ability to match the power transistor to the sensing transistor. It also has limited accuracy because the resistance of the transistors typically change over time, so measuring the drain/source voltage to obtain current typically will not be accurate over time.
Some power transistors are fabricated using laterally diffused metal oxide semiconductor (LDMOS) techniques that change characteristics with time if they are switched with high voltage on their drains. In some embodiments, the change in characteristics happens quickly at first then tapers off with time, resulting in a change that is a logarithmic function of time. Other embodiments have different aging profiles that are not easily predicted. For example, baking LDMOS devices or operating the devices in different modes can reverse or accelerate changes in characteristics. These changes present problems with devices used in SenseFET technology because a change in the current ratio leads to inaccurate current sensing.